Hebrews 5:14
But solid food is for those who are full grown, who by reason of use have their senses exercised to discern good and evil.
WEB
But strong meat belongeth to them that are of full age, even those who by reason of use have their senses exercised to discern both good and evil.
KJV
What Hebrews 5:14 means
It says spiritual maturity looks like trained judgment: a sense for good and evil that has been sharpened by long practice, the way athletes build strength.
The unknown author of Hebrews is pausing his argument to scold his readers. By now they should be teaching others, he says in verse 12, yet they still need milk, the beginner's diet. Milk is for infants; solid food belongs to the grown.
What marks the grown is the striking part: their senses have been "exercised," a word borrowed from athletic training. In this picture discernment works like a muscle. It grows in people who keep using it, choice after choice, until telling good from evil becomes almost a reflex. Maturity here is measured by practice, however old the believer happens to be.